An object of the present invention is to be able to control the position and the size of the focal spot of the X-ray tube in an X-ray equipment for diagnostics which normally is provided with an X-ray tube with rotary anode, but the technique according to the invention can also be utilized in other equipment. By determination of the size of the focal spot of an X-ray tube, a method according to IEC 336 is normally used. By means of a slit-camera a longish image is achieved which constitutes a projection of the extension of the focal spot in one direction, the image of which is recorded photographically and can be analyzed by a micro-densitometer. This is a time-consuming and circumstantial procedure, which in addition can be marred by unnecessary error sources. When measuring on weak X-ray generators which are utilized for instance in connection with mammography, a large number of exposures have to be made to obtain sufficient density on a film of the image through the slit. Differences in the position of the X-ray tube and of the measuring equipment at the various exposures may occur, whereby the image does not become correct. Moreover, only this part of the measurement becomes time-consuming, thereafter time has to be included for development and read-out of the image.